Germanium-helium alloys and rectifiers made therefrom



Aug. 24, 1948. R. M. WHALEY 2,447,829

GERIIANIUll-HELIUII ALLOYS AND asc'rmmns was THEREFROI Filed Aug. 14, 1946 2 Sheets-Sheet 1 Vans (Ge NEGATIVE) SQUQJNHITIIW GERMflN/UM MEL T 21 V MELTED IN HEL/uM/vo OTHER IMPURITY ADDED BEFORE HEHT TREHTMENT HFTER 500C 18 HRS. HEHT TREH TMENT m mcuuM.

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R. M. WHALEY GERIANIUH-HELIUI ALLOYS AND RECTIFIERS HADE THEREFROM 2 Sheets-Sheet 2 Filed Aug. 14, 1946 mnewuvewowerowewguwawflowiwoww E; k Gem E 2 4 o fiandzafiz i zm zg BY I mam/ Patented Aug. 24, 1948 GERMANIUM-HELIUM ALLOYS AND RECTI- FIERS MADE THEREFROM Randall M. Whaley, West Lafayette, Ind., assignor to Purdue Research Foundation, Lafayette, Ind., a corporation of Indiana Application August 14, 1946. Serial No. 690,545

14 Claims. (Cl. 175366) My present invention relates to an improve-:

ment in alloys of germanium, method of making the same, and to rectifiers of electricity made of such alloys.

In the copending application of Karl Lark- Horovitz and myself, Serial No. 604,744, filed July 13, 1945, there is disclosed a large number ofalloys of germanium having utility as semiconductors for contact type rectifiers, and more particularly for semi-conductors of N-type which present high resistance to current flow across the rectifying contact when the semi-conductor is positive and the contacting metal electrode or whisker is negative and a lower resistance when the potential is reversed.

My present invention is concerned with further alloys of germanium having utility for the purpose above noted. According to my present invention germanium is impregnated with helium in view of which the term alloy is not used in its common sense as meaning the union of two or more metals. However, for purposes of the instant disclosure the words alloy of germanium as used herein means to include the impregnation of germanium with helium and the combination of which exhibits electrical properties such as are found in metals and semi-conductors.

In the above referred to application numerous disadvantages of contact rectifiers of the prior art such as those having semi-conductors made of galena, silicon, iron pyrites or other material are set forth. The foremost disadvantages of such rectifiers are their inability of withstanding in continuous use voltages in the high resistance direction greater than a few volts without permanent injury or destruction of the rectifier and for some the resistance in the high resistance direction is not sufiicient to enable use thereof inhigh impedance circuits, that is, circuits over several thousand ohms. Further, some of such rectifiers do not function to pass sumcient current in the vention before and after heat treatment of the ized by a negative resistance, l. e., the voltage across the rectifying contact drops upon. increase in current flow therethrough after the peak back voltage is reached.

Other features and advantages of my invention will appear from the detail description.

Now, in order to acquaint those skilled in the art with the manner of making alloys in accordance with my invention and the utilization thereof as rectifiers of electricity, I shall describe in connection with the accompanying drawings and the tables following hereafter certain of the precesses used in making the alloys which lie withinmy invention.

In the drawings:

Figure 1 is a graph illustrating representative voltage-current characteristic curves of a rectifler the semi-conductor element of which is made of an alloy of my present invention with the curves shown in solid lines indicating the characteristics of selected points on the semi-conductor before heat treatment of the alloy, and the curves shown in broken lines indicating the characteristics of selected points on the semi-conductor after heat treatment of the alloy in vacuum at 500. C. for 18 hours;

Figure 2 is a graph similar to that of Figure 1 and illustrating representative voltage-current characteristics of selected points of contact of a semi-conductor made of another alloy of my inalloy; and

Figure 3 is a perspective view of a typical form of contact type rectifier, the semi-conductor of which is made of an alloy of my invention.

The typical voltage-current characteristic curves of Figure l are of an alloy identified as melt 21V, which alloy was prepared by melting germanium in an atmosphere of helium. The

low resistance direction for satisfactory operation of associated apparatus.

The alloys of my present invention like those of the above referred to applicationhave utility for use as N-type semi-conductors incontact type rectifiers and when so employed exhibit high back resistances and low forward resistances. Further, with semi-conductors of the alloys of my invention rectifiers may be made which are capable of withstanding high voltages in the back direction of the order of 50 to 200 volts. After the voltage peak in the back or high resistance direction is reached, the rectifiers are characterimpregnating of the germanium with helium may be accomplished in any-desired manner, as for example, by melting pure germanium at a temperature above its melting point of 960 C. to about 1050 C. in a boat or crucible disposed within a suitable furnace. The germanium is then maintained in molten stage in an atmosphere of helium in the furnace for a period ranging from 5 to 15 minutes. The helium may be admitted into the furnace until approximately atmospheric pressure is reached and if desired the system may then be closed 01! to effect the union of the germanium and the helium. However, if i the uniting of germanium with the helium produced satisfactory melts but no indications were observed in the carrying out of the above procedure that the time of melting or the temperature range were critical. In preparing the melts, however, precaution should be taken to prevent the accidental introduction of unknown and perhaps detrimental impurities into the melt such as from the crucible or boat in which the germanium is disposed for melting, the furnace itself, or some material volatillzed in the furnace. The presence of small traces of unknown impurities in the germanium should be avoided, and in the melts satisfactorliy produced in accordance with my invention the germanium had a purity approaching 100%, and an electrical resistivity greater than about 1 ohm cm.

After the germanium and helium have been allowed to mix the melt is allowed'to solidify and cool which may be accomplished either by immediately removing the heat or by controlled cooling apparatus. Melts of 5 to 6 grams prepared in the above manner were cut into wafers and after grinding and etching in the manner set out in detail in the above application and to be hereinafter referred to, 'were embodied as the semi-conductor in a contact type rectifier. The selected points of contact between the electrode or whisker with the prepared surface of the semiconductor produced the current-voltage characteristics shown in full lines in Figure 1, and will be referred to in great detail hereinafter.

As mentioned above the contact surface of the alloy is preferably first ground flat and then etched. Any suitable grinding composition may be used, as for example, 600 mesh alumina. How

ever, etching of the alloy is not essential since As above referred to andlas disclosed in detail in the above copending application of Lark- Horovitz and myself to which reference may be had, a suitable etching solution for the alloy may comprise the following composition:

. 4 parts by volume hydrochloricacid (48 reagent) Such a solution will satisfactorily etch the ground contact surface of the wafers in about one to two minutes at room temperature and may be applied with either a swab or by immersing the surface in the solution. The etching is not particularly critical but care should be taken not to unduly extend the etching since airigh polish may be produced impairing the alloy. 0

Also I have disclosed in the above referred to application other types of satisfactory etching solutions and techniques. Such alternate etching solutions and procedures are as follows:

A solution consisting approximately of 1 gram stannyl chloride in 50 cc. of H20 may be used as an electrolytic bath for etching the alloy surface. Immersing the alloy as the anode in this solution will result in satisfactory etch within about 1% minutes at about 2% volts applied.

Also an electrolytic solution comprising 5 parts 4 concentrated BNO: and 50 parts 5:0 by volume, and using the alloy as the anode for about 1% minutes at 1 to 2 volts has proven satisfactory.

After completion of the grinding and etching of the alloy above identified as melt 21V, it was subjected to heat treatment in vacuum at a temperature of about 500 C. for 18 hours. After heat treatment of the melt 21V and reassembly thereof, in a point contact type rectifier, the current-voltage characteristics or selected points on the surface are illustrated by the broken line curves of Figure 1. In general it will be observed that in melt 21V, and before the last referred to heat treatment, that the voltage-current characteristics are nearly symmetricwith respect to the origin, and after heat treating of the melt 21V, above described, the currentvoltage characteristics were radically changed. In Figure 1 it will be observed that melt 2 IV after heat treatment provided a semi-conductor of t e N -type, that is a semi-conductor which when assembled into a contact type rectifier presents a high resistance to current flow across the rectifying contact when the semi-conductor is positive and the contacting metal electrode or whisker is negative, and a lower resistance when the potential is reversed, Further, the resistance in th forward or low resistance direction is considerably reduced. Prior to the heat treatment the melt 21V, as illustrated by the solid line curves of Figure 1, exhibits high resistances in both directions and substantially ohmic conduction with a tendency toward P-type conductivity. As such, the alloy does not produce good crystal rectiflers, i. e., good in the sense of having low forward resistances and high back resist.- ances. However, after the heat treatment step the alloys exhibited N-type behavior and as illustrated by the broken line curves were characterized by low forward resistances and high back resistances. In rectiflers made of the melt 21V and as indicated by the broken line curves. it will be observed that the rectifier withstood high voltages in the back or high resistance direction of the order of 50 to 200 volts before the voltage-current characteristics passes over into a region of negative resistance upon further increase of current.

In Table I below I have set forth the values of selected points of contact of melt 21V shown by' I Vp, the current at the voltage peaks is set forth under the legend I; and the current at one volt under the legend II.

Tenn: I

Melt 21V-Pure germanium melt in helium Germanium Germanium Positive Negative Vp I II In Ii l0 l5 1 40 20 l 20 25 l 50 i7 l 15 25 l. 4 35 20 1. 4 15 15 l. 2 35 15 l. 2 i5 15 l. 4 30 25 l. 4 55 10 2 30 15 6 35 12 6 10 10 l. 8 30 2. 5 1. 6 15 20 8 4D 20 8 15 25 l. 2 35 20 1. 2 20 12 l 55 9 1 l5 8 l 45 13 1 18 12 l 40 25 l TAM-VIII- Melt '21V-Ajter heat treatment in 500 C. for 18 hours vacuum at Germanium Germanium I Negative Positive can In Figure 2 I have shown characteristic current-voltage curves of another alloy of my invention. The alloy exhibiting the electrical characteristics of this figure is identified as melt 34V. This melt was prepared in substantially the same manner 'as the germanium-helium alloy previously described except that the ingredients disposed in theboat consisted of relatively pure germanium together with .1 atomic percent tin, i. e.. tin was added in' the proportionate number of atoms in percent of the total number atoms of germanium and tin present. The germanium and tin was then melted in an atmosphere of helium as set forth in describing the preparation of melt 21V above. Upon solidification, the melt is preferably ground and etched in accordance with one of the abovefrelate'd procedures. Upon assembly oi the alloy asfa'semi-conductor in a rectifier selected-points of contact of the surface of the semi-conductor with a whisker revealed the current-voltage characteristics shown by the solid line curvesof Figure 2.

The following Table III sets forth the values of the current-voltage characteristics of melt 34D prior to heat treatment.

Thereafter the alloy was heat treated in vacuum at about 500 C. for 19 hours. In Table IV below there is set forth the current-voltage characteristic of melt 34D after heat treatment of the alloy.

Trust: IV

Melt 34D.1% Sn melted in helium after heat treatment in vacuum at 500 degrees C. for 19 'hrs.

Germanium Germanium Positive Negative VD n 1| b Ii 110 5 Y Mn 4 150 4 5 120 5 4 In Figure 2 the broken line curves illustrate the current-voltage characteristics after heat treatment of melt 34D. I

In connection with the data of Tables III and IV and of the curves of Figure 2, I wish to point out that germanium-tin alloys described in the aforementioned copending applicationot Lark- Horovitz and Whaley usually exhibit a concenalloy, but that in germanium or germanium al-' tration gradient of tin within the alloyso that certain regions of the alloy are suitable for production of rectiflers resulting from the presence of tin in sufllcient amounts. in those regions. Other regions, comprising a relatively smaller fraction of the total melt, contain only a negligible amount of tin .and thereby produce voltagecurrent characteristics when assembled into rectifiers as would result from the use of pure germanium melted in vacuum. The tin. therefore, is present in negligible amounts in such regions, which are the regions selected for tests described in Figure 2 and for Tables III and IV. Whatever conductivity results or eflects follow heat treatment are attributable to the presence or absence of helium during the melting operation.

In tests conducted by me in connection with heat treatment or pure germanium melted in vacuum it was observed that heat treatment eifected substantially no change in the voltagecurrent characteristics observed, and were substantially symmetric with respect to the origin, but with some characteristics actually having higher peak voltages when the germanium'was negative than when it was positive indicating P- type conduction. In generalthe characteristics of heat treated germanium melted in vacuum were high resistance in both directions of current flow, with the heat treatment not having any substantial eil'ect on readily observable electrical characteristics.

In further work on the heat treatment of germanium alloys a melt identified as melt 18T comprising germanium and .1 atomic percent tin was melted in high vacuum and contact points selected in tin-tree regions of the alloy point exhibited voltage-current characteristics similar in most respect to those found on pure melts oi germanium melted in vacuum. Subsequent-heat treatment of such an alloy at 500 C. in vacuum for 18 hours produced substantially no change in the voltage-current characteristics of rectiflers made with the same tin-free regions of the melt.

From these experiments the conclusion may be drawn that the mere heat treatment of germanium or germanium alloys, excluding the alloys of my present invention, in vacuum eilfects no change in the electrical characteristics of the loys having impurity-tree regions thereof impregnated with helium, when subjected to the heat treatment herein disclosed, provides an alloy exhibiting N-type semi-conductor characteristics in view of which the alloys have utility for use in rectifiers.

Referring now to Figure 3 I have shown a typical form of contact rectifier having a semiconductor made of an alloy of my invention. Referring to Figure 3 it will be observed that the rectifier comprises a semi-conductor 3 made of a germanium alloy of my present invention. The semi-conductor 3 is preferably soldered to a conductor plate 4 of any suitable metal or alloy having good electrical conductivity characteristics.-

An electrode or whisker 5 is formed with a pointed end which makes point contact with the surface of the semi-conductor 3. The electrode or whisker 5 is made of a metal or alloy having good electrical conductivity and preferably of good mechanical strength. Some of the metals that have been found satisfactory for use in making the electrode or whisker are tungsten, platinum, copper, iron, gold, silver, manganese, pantalum, nickel, zinc, molybdenum, zirconium, lead and platinum-iridium alloy. The electrode 5 preferably is provided with a sharp pointed end having a tip diameter of the order of 0.001". These electrodes may be readily made by forming suitable points on wires of about 0.005" in diameter. A lead 6 is suitably connected to the electrode 5 and lead I had connection with the conductor plate 4 upon which the semi-conductor 3 is mounted. Construction of a device as indicated in Figure 3 with the semi-conductor element 3 made of the alloys of my invention will provide devices the characteristics of which are indicated in the graphs of Figures 1 and 2 and set forth in Tables I through IV.

It is not'known what eflect the helium has in the germanium and more particularly after heat treatment of the germanium but it is thought that the helium effects ionization in some respect or changes the lattice structure of the germanium providing for the electrical characteristics above related. Also in the impregnating of germanium with helium it is not known to whatextent the germanium takes up the helium or if t is the helium that is responsible for the electrical effects or if they are caused by some impurity present in commercial helium. However, by following the above disclosure the alloys of my present invention may be readily produced.

I claim:

1. An alloy comprising germanium impregnated with helium.

2. An alloy comprising germanium impregnated with helium, the alloy being characterized by exhibiting the electrical property of high resistance to current flow in one direction therethrough and low resistance to current flow in the other direction therethrough.

3. An alloy comprising germanium impregnated with helium and subjected to heat treatment subsequent to formation, the alloy being characterized by exhibiting the electrical property of high resistance to current flow in one direction therethrough and low resistance to current flow in the other direction therethrough.

4. An alloy obtained by heat treating germanium impregnated with helium in vacuum at about 500 C. for about 18 hours.

5. An alloy obtained by maintaining germanium at the molten state at a temperature range of about 960 C. to 1050" C. in the presence of helium for about 5 to 15 minutes to impregnate the germanium with helium, and after solidification'thereof heat treating the same in vacuum at 500 C. for about 18 hours.

6. An alloy comprising germanium impregnated with helium and subjected to heat treatment subsequent to formation, the alloy being characterized by exhibiting the electrical property of high resistance to current flow therethrough when of positive potential and of low resistance to current flow therethrough when of negative potential.

7. The method of making a semi-conductor for a rectifier comprising impregnating germanium with helium.

8. The method of making a semi-conductor for a rectifier comprising forming an alloy byimpregnating germanium with helium, and then heat treating the impregnated germanium alloy at a temperature of about 500 C. for about 18 hours.

9. The method of making a semi-conductor for a rectifier comprising melting germanium in hellum at a temperature of 960 C. to 1050 C. for about 5 to 15 minutes to impregnate the germanium with helium and after solidification thereof heating the same in vacuum at about 500 C. for about 18 hours. 10. A rectifier comprising a semi-conductor, and an electrode having contact with a surface of said semi-conductor, said semi-conductor comprising an alloy of germanium and helium.

11. A rectifier comprising a semi-conductor, and an electrode having contact with a surface of said semi-conductor, said semi-conductor comprising an alloy of germanium and helium heat treated to provide the electrical characteristic of high resistance to current flow in one direction therethrough and low resistance to current flow in the other direction therethrough.

12. A rectifier comprising a semi-conductor, and an electrode having contact with a surface of said semi-conductor, said semi-conductor comprising an alloy obtained by heat treating germanium impregnated with helium in vacuumat about 500 C. for about 18 hours.

13. A rectifier comprising a semi-conductor, and an electrode having contact with a surface of said semi-conductor, said semi-conductor comprising an alloy obtained by melting germanium at a temperature range of 960 C. to 1050 C. in the presence of helium for about 5 to 15 minutes to impregnate the germanium with helium, and after solidification thereof heat treating the same in vacuum at 500 C. for about 18 hours.

14. A rectifier comprising a semi-conductor, and an electrode having contact with a surface of said semi-conductor, said semi-conductor comprising an alloy of germanium and helium heat treated to provide the electrical property of high resistance to current flow therethrough when of positive potential and of low resistance to current flow therethrough when of negative potential.

RANDALL M. WHAIEY. 

